JOIN is used to combine rows from two or more tables based on a related column, while UNION is used to combine the result sets of two or more SELECT statements into a single result set. JOIN is more suitable for retrieving related data from multiple tables, while UNION is better for combining similar data from multiple tables or result sets.
See related: Inner Join vs Outer Join.
Comparison chart
 | Join (SQL) | Union (SQL) |
|---|---|---|
| Introduction (from Wikipedia) | A SQL join clause combines columns from one or more tables in a relational database. It creates a set that can be saved as a table or used as it is. A JOIN is a means for combining columns from one or more tables by using values common to each. | In SQL, UNION is an operator that combines the result sets of two or more SELECT statements into a single result set. The UNION operator is used to merge the rows returned by the SELECT statements, effectively concatenating the results vertically. |
| Purpose | Combines rows from two or more tables based on a related column between them. | Combines the result sets of two or more SELECT statements into a single result set. |
| Syntax | SELECT columns FROM table1 JOIN table2 ON table1.column = table2.column; | SELECT columns FROM table1 UNION SELECT columns FROM table2; |
| Type of operation | Joins tables horizontally | Combines tables or result sets vertically |
| Result | Returns a result set containing columns from both tables, filtered based on join conditions | Returns a result set combining rows from both SELECT statements, removing duplicates by default |
| Number of tables | Combines data from two or more tables. A self-join allows you to combine row from two "virtual instances" of the same table. | Combines result sets from two or more SELECT statements, which can be from the same or different tables. |
| Number of columns | The number of columns in the result set depends on the columns selected in the query. | The number and order of columns must be the same in all SELECT statements whose results are being combined by the UNION. |
| Duplicate rows | Keeps duplicate rows from the joined tables. No de-duplication is done. | Removes duplicate rows from the combined result set (use UNION ALL to keep duplicates). |
| Matching rows | Returns only the rows that have matching values in the specified columns (INNER JOIN). | Returns all rows from both SELECT statements, regardless of matching values. |
| Performance | Can be faster for large datasets, as it only combines the necessary rows. | Can be slower for large datasets, as it combines all rows and then removes duplicates (if UNION is used). |
| Use cases | Retrieving related data from multiple tables. | Combining similar data from multiple tables or result sets into a single result set. |
Understanding UNION in SQL
In SQL, UNION is an operator that combines the result sets of two or more SELECT statements into a single result set. The UNION operator is used to merge the rows returned by the SELECT statements, effectively concatenating the results vertically.
The basic syntax for using UNION is as follows:
SELECT column1, column2, ... FROM table1 UNION SELECT column1, column2, ... FROM table2;
Here are some key points about UNION:
- The SELECT statements within the UNION must have the same number of columns in their result sets.
- The columns in each SELECT statement must have similar data types.
- The column names in the result set will be taken from the column names in the first SELECT statement.
- By default, UNION removes duplicate rows from the combined result set. If you want to include duplicate rows, you can use UNION ALL instead.
UNION is useful in scenarios where you want to combine similar data from multiple tables or result sets into a single result set. Some common use cases include:
- Combining data from multiple tables that have similar structures.
- Merging result sets from different SELECT statements to create a comprehensive report.
- Performing set operations like combining distinct values from multiple columns.
It is important to note that the UNION operator performs a distinct operation, meaning it removes duplicate rows from the combined result set. If you want to retain all rows, including duplicates, you can use UNION ALL instead.
UNION provides a convenient way to vertically concatenate result sets and combine data from multiple sources in SQL.
Understanding JOIN in SQL
JOIN is a clause that is used to combine rows from two or more tables based on a related column between them. JOIN allows you to retrieve data that is spread across multiple tables by establishing a relationship between the tables using a common column.
The basic syntax for using JOIN is as follows:
SELECT columns FROM table1 JOIN table2 ON table1.column = table2.column;
Here are some key points about JOIN:
- JOIN is used to combine rows from two or more tables based on a related column.
- The most common type of JOIN is INNER JOIN, which returns only the rows that have matching values in the specified columns of both tables.
- There are other types of JOINs, such as LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN, which return different combinations of rows based on the presence or absence of matching values.
- The ON clause specifies the condition for joining the tables, typically by comparing a column from one table with a column from another table.
- The columns used in the JOIN condition should have a logical relationship or a foreign key-primary key relationship between the tables.
JOIN is essential when you need to retrieve related data from multiple tables. Some common use cases include:
- Combining data from related tables to generate reports or perform analysis.
- Retrieving data from a main table along with associated data from related tables.
- Implementing master-detail relationships or many-to-many relationships in a database.
When using JOIN, it's crucial to understand the relationship between the tables and choose the appropriate type of JOIN based on your requirements. INNER JOIN is the most commonly used type and returns only the rows that have matching values in both tables. LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN are used when you want to include rows from one or both tables, even if there are no matching values in the other table. See a detailed comparison of the different types of JOINs in SQL.
By utilizing JOINs effectively, you can combine data from multiple tables and retrieve meaningful information from your database.
References
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